1. Field of the Invention
The present invention relates to a process and apparatus for manufacturing an electromagnetic interference shielding metallic foil cladded plastic product, and more particularly to a process and apparatus for manufacturing an electromagnetic interference shielding metallic foil cladded plastic product by using two techniques, the superplastic forming of a superplastic alloy and the injecting of softened plastic.
2. Description of the Prior Art
In recent years, progress in technology has led to an extensive increase in the amount of sophisticated electronic equipment. However, the high-density electromagnetic waves produced from electronic equipment have the risk to damage or adversely affect the performance of other equipment or components. Also, exposure to electromagnetic waves is harmful to the human body. Therefore, an electrically conductive outer shell is needed to shield electromagnetic interference (EMI) produced from electronic equipment.
Heretofore, various methods have been used to shield electronic equipment. Metallic boxes and cans fabricated from steel, copper, aluminum, etc., were used to surround high EMI emitters as shielding. However, metallic shields with intricate shapes were difficult to be fabricated by the conventional metalworking methods. Moreover, metallic shields were cumbersome, heavy and costly. Therefore, the electronic industry has resorted to metallized plating on plastics. Unfortunately, the results obtained with metallic coatings were not always satisfactory. In addition to being relatively non-economical, once such metallic coatings were scratched through, they would lose part of their shielding efficiency. Unless such conductive coatings are continuous and free of voids, electromagnetic waves will be free to pass through. Frequently, it was difficult to obtain a dependable, 100% effective coating which was also resistant to peeling.
Further efforts by the electronics industry to develop more dependable light-weight materials for EMI shielding have led to a third approach, namely electrically conductive component-filled plastic composites. It was anticipated that intricate shapes could be molded from the composite materials by conventional means, yielding a finished part that promised to be more economical and dependable than metal or metal-coated plastics.
The principle factor influencing the performance of conductive component-filled plastic composites is the aspect ratio of the conductive fillers. The aspect ratio is defined as the ratio of the maximum dimension to the minimum dimension of the filler. For example, the aspect ratio of a fiber is the ratio of the length to the diameter of the fiber. According to the electromagnetic wave percolation theory, if the conductive filler in the plastic retains a higher aspect ratio, the filler easily forms a conductive network, thus, the critical concentration of the conductive filler required to achieve the electromagnetic shielding effect (that is, the threshold percolation concentration) is lower.
The methods for preparing conductive component-filled plastic composites can be classified into three types. The first type involves compounding the conductive fillers in the form of powders, short fibers or flakes with the plastic matrix, and then having the mixture hot-press molded or injection molded into various kinds of plastic products for shielding EMI.
For example, U.S. Pat. No. 4,474,685 discloses a process for fabricating electromagnetic shielding products by first compounding and then molding a composite including a thermosetting resin binder and an electrically conductive filler (including carbon black, graphite and conductive metal powders). However, during the compounding with the resin matrix, the conductive powders may easily cluster, and thus are not capable of dispersing in the resin matrix. Consequently, the electromagnetic shielding efficiency of the molded products can not be effectively improved. Furthermore, since the powder filler has a lower aspect ratio, according to the electromagnetic wave percolation theory as mentioned above, the amount (i.e., threshold percolation concentration) of the powder filler added must be relatively high to achieve electrical conductivity. Consequently, the mechanical properties, color and other physical and chemical properties of the molded products are adversely affected.
On the other hand, if the conductive filler is in a higher aspect ratio form such as fibers or flakes, the filler can be loaded to a lower level. However, the cluster phenomenon is still difficult to prevent. In addition, during the compounding process, in order to maintain the original aspect ratio, the conductive filler should be strong enough to prevent breakage due to compounding. However, such a strong conductive filler is very expensive, and is thus not suitable for ordinary low cost electronic equipment. More seriously, when the resultant plastic pellets containing such conductive filler with high strength are subjected to injection molding, the mold, screws and compressing cylinder walls of the injection machine will suffer extensive wear.
The second type of method for preparing conductive component-filled plastic composites involves binding a plastic layer to enclose the conductive continuous filler by immersion or extrusion, and then cutting the conductive long fiber-filled plastic stick to a predetermined length. For example, Japanese Patent No. 60-112854 discloses a process including continuous extruding thermoplastic plastic to enclose a copper fiber to form a copper fiber-filled plastic round stick, and then cutting the plastic round stick into pellets of a predetermined size. In order to increase the aspect ratio of the filler, the diameter of the conductive long fiber should be as small as possible. The fibrous filler must be strong enough to prevent breakage. Because the compounding step is omitted in the second method, clustering of the filler is improved. However, again, when the resultant plastic pellets containing a conductive filler with high strength are subjected to injection molding, the mold, screws and compressing cylinder walls of the injection machine will suffer extensive wear.
The third type of method for preparing conductive component-filled plastic composites involves sandwiching an electrically conductive metal foil in between two plastic films to form a metallized laminated plastic sheet; slicing the plastic sheet into plastic strips; radially arranging the metallized plastic strips into a die of an extruder to be wetted and bound by softened plastic into a metallized plastic bar; and finally cutting the plastic bar into metallized plastic pellets of a predetermined size, which has been disclosed in U.S. Pat. No. 5,531,851 and German Patent DE 19517554C2.
In the third method, the compounding step is omitted, thus clustering of the filler is improved. In addition, the metal foil in the plastic/metal/plastic laminated strips is reinforced by plastic. Therefore, even aluminum foil with lower strength is applicable. Hence, when the resultant plastic pellets are subjected to injection molding, the wearing of the mold, screws and compressing cylinder walls of the injection machine will be lessened. However, in the injection molding step, such aluminum foil with lower strength has a higher possibility of breaking. Therefore, there is a need to use a specially designed injection screw and injection mold.
The above three methods all involve subjecting the conductive pellets to hot-pressing or injection molding to obtain the final plastic article for shielding EMI. When injection molding is employed, in order to prevent lag, segregation, and orientional phenomena from occurring to the metallic filler in the mold, the design of the mold is very critical and complicated, thus increasing costs. Moreover, if a very thin product is desired, such as the outer shell of a notebook computer, the design of the mold is difficult, or even impossible to achieve.
In addition, floating phenomenon in the final conductive plastic article can not be prevented. Therefore, surface plastic coating on the final conductive plastic article is required, thereby increasing cost. Most importantly, according to the above three methods, the conductive filler in the plastic is not continuous (i.e. non-solid). Therefore, the resulting electromagnetic shielding effectiveness of such plastic article is limited and far less than that of a conventional metal outer shell.
Generally speaking, the shielding effectiveness of a conventional metal plate can reach 80 dB or more, which meets the requirement for various electronic equipment in various countries (FCC for U.S.A., VDE for Germany, VCCI for Japan, CSA for Canada, CISPR for Russia). However, a plastic article containing discontinuous (non-solid) conductive filler obtained from the above three methods has a shielding effectiveness of only about 50 dB, which can only meet the requirements for simple electronic equipment (40 dB), such as personal computers.
Nowadays, personal computers are usually equipped with CD-ROM, LSI, or other circuits of high density and high frequency. Therefore, in the future, it is anticipated that personal computers will require a higher shielding effectiveness, for example, 60 dB or more. Obviously, the plastic articles obtained from the above three methods will not meet the future needs. Furthermore, the conductive component-filled plastic articles obtained from the above three methods obviously can not be used for shielding the electronic equipment which needs high shielding effectiveness, such as work stations, remote control systems, cellular phones, and notebook computers.
Therefore, there is a need to provide a process for manufacturing a metallized plastic article having high shielding effectiveness (e.g., higher than 60 dB).